[PLAT-9215] Move to threadsx

[aeischeid]

Summary

Migrates the web-worker packages from unmaintained threads to the maintained threadsx fork.

• Replaces threads imports and peer dependencies with threadsx@^2
• Adds Node 20+ engine metadata and explicit ESM/CJS export maps
• Documents consumer migration steps
• Fixes spawnPool() defaults and standardizes pool termination options as { terminate }

Alternatively I think we could also mark this repo as deprecated. The benefits of this wrapper are largely obsolete in threadsx, and effort to move to a new version of web-workers with threadsx peerDependency would likely be better spent just moving to threadsx straight away as in Vertexvis/vertex-web-sdk#837

Test Plan

explore usage in web-sdk and

Release Notes

see readme

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud